1. Field of the Invention
The invention relates to a composite material having a high refractive index, its production process, as well as optically active materials such as antireflection and reflecting materials made from said composite material.
2. Description of the Prior Art
Antireflection and reflecting materials are formed by an organic or inorganic substrate, covered with several coatings, certain of which have sought, specific, optical properties. More specifically, interferential dielectric mirrors incorporating a substrate, covered with a dielectric film reflecting one or more desired wavelengths, whilst having a relatively low intrinsic absorption compared with the metals conventionally used for producing such mirrors. Antireflection or reflecting materials have numerous applications.
Thus, organic or inorganic substrates, i.e. in particular plastics or vitreous substrates, coated with an antireflection film have particular interest in the fields of ophthalmic and video products or architectural applications, such as glass panels placed on the outside of buildings. In addition, antireflection materials and interferential dielectric mirrors can also be used in high energy lasers, solar, thermal and photovoltaic applications or in integrated optical systems.
The prior art already discloses processes making it possible to produce antireflection materials or interferential dielectric mirrors and said processes are referred to hereinafter.
Thus, U.S. Pat. No. 2,466,119 discloses a process for the preparation of multilayer, reflecting and/or antireflection films, by hydrolysis and condensation of mixtures of titanium halides and/or silicon alkoxides. The porosity of said layers is controlled by varying the temperature. However, the obtaining of layers having a good mechanical strength requires heating to temperatures above those which can be withstood by conventional plastics, whose maximum thermal stability is 150.degree. C.
The NTIS U.S. patent application 7 148 458 corresponding to U.S. Pat. Nos. 4,929,278 and 4,699,812, describes a process for depositing antireflecton films on plastic substrates consisting of synthesizing an ethanol gel in the SiO.sub.2 --B.sub.2 O.sub.3 --Al.sub.2 O--BaO system until a certain molecular complexity is obtained, followed by the reliquefying of said gel by mechanically breaking certain interpolymer bridges. This gives a porous film with a low refractive index of approximately 1.23, produced at ambient temperature, which permits an adaptation to plastic substrates. However, this film only has a mediocre abrasion resistance.
U.S. Pat. Nos. 2,432,484 and 4,271,210 disclose the possibility of using silica or alumina colloids for producing antireflection dielectric coatings, making it possible to increase the porosity of said coatings and therefore lower their refractive indexes. However, the colloidal coatings obtained have a very poor mechanical strength and are particularly sensitive to any physical contact.
Moreover, although in the solar sector, plastics such as polycarbonates, polyacrylates, polyallyl carbonates, etc. are particularly interesting, vitreous substrates are also of interest, particularly in the field of general optics. However, it is clear that due to approximately 4% reflection losses for each air-glass interface encountered (the average index of glass being 1.5), the balance of losses for a complex optical system is often disadvantageous.
Therefore optical experts have long sought to create antireflection films by the use of physical processes such as vacuum deposition or evaporation. However, these processes are sophisticated and expensive, not generally being adapted to large scale, inexpensive production.
Finally, French patent application FR-A-2 680 583 of the CEA describes a material having antireflection properties, as well as abrasion resisting and hydrophobic properties. This material comprises an organic or inorganic substrate, successively covered by an adhesion promoting coating made from a material chosen from among silanes, an antireflection coating of silica colloids coated with a siloxane binder, a coupling agent coating formed from a material chosen from among the silazanes and an antiabrasive coating of a fluorine polymer. However, this material has a relatively small spectral transmission window of only about 100 nm.
The prior art includes documents describing more specifically materials having reflecting properties, as well as their production process.
U.S. Pat. No. 3,460,956 describes the preparation of TiO.sub.2 reflecting films obtained from tetraalkyl titanate hydrolyzates in an alcoholic medium. However, for an effective conversion of the polymer film into dense oxide, said film must undergo heating at high temperatures of about 500.degree. C. and which are therefore prejudicial to the entire organic substrate.
U.S. Pat. No. 2,710,267 describes the production of reflecting TiO.sub.2 films from alcoholic sols of a titanium alkoxide, said sols being hydrolyzable by atmospheric moisture. However, the coatings obtained are not abrasion resistant.
The CEA French patent application 2 682 486 describes the preparation of dielectric mirrors having a high laser flux resistance using a process performed at ambient temperature, which permits an adaptation to organic substrates. The films having the desired optical properties are prepared from colloidal suspensions, which are deposited by alternating a material with a low refractive index with a material having a high refractive index.
However, the colloidal coatings used are by their very nature porous, which leads to a low refractive index, compared with that of the same material in dense form. Consequently, for an equivalent reflectivity, it is necessary to stack a larger number of coatings in order to obviate this index difference, which increases the production times and costs.
In order to meet the deficiencies or defects of the products described hereinbefore, the invention consists of the development of a novel material having a high refractive index and the use thereof in the production, at ordinary temperature, of optically active materials. The latter are e.g. either materials having antireflection properties over a wide spectral band and good water repelling properties and abrasion resistance properties, or reflecting materials which may or may not have e.g. abrasion resisting properties.